As discussed in U.S. Pat. No. 5,179,420 (So), for example, in semiconductor-based, short-cavity lasers, such as Fabry-Perot-type (FP) lasers and distributed feed back (DFB) lasers, photon residence time is relatively short, so they are capable of producing pulses of short duration, e.g., <1 ns. Moreover, they can produce pulses of variable duration (<1 ns to continuous wave) and with an arbitrary pulse repetition rate. Consequently, these lasers have been preferred for use with commercially-available optical time domain reflectometers (OTDRs), instruments which are capable of launching a series of pulses of various durations and repetition rate, making them suitable for characterizing optical fibers in optical communications networks.
In a paper entitled “Analysis and design of Q-switched erbium-doped fiber lasers and their application to OTDR”, J. of Lightwave Technology, 20(8), pp. 1506-1511 (2002), S. Adachi, Y. Koyamada explained that longer cavity lasers, such as fiber ring lasers or extended linear-cavity lasers, despite their long photon residence time, can also produce narrow pulses when operated in mode-locked or Q-switched operation, but they cannot readily produce both very short pulses and pulses of variable duration or arbitrarily variable repetition rate, and hence have hitherto been considered ill-suited for most OTDR applications.
It is possible to produce fast rise-time short light pulses for such long cavity lasers by using an external optical modulator to modulate the CW light of the long cavity laser and an optical amplifier to compensate the loss from the modulator, as disclosed by A. Rossaro, M. Schiano, T. Tambosso, D. D'Alessandro, “Spatially resolved chromatic dispersion measurement by a bidirectional OTDR technique”, IEEE J. of Selected Topics in Quantum Electronics, 7(3), pp. 475-483 (2001) and by H. Sunnerud, B.-E. Olsson, P. A. Andrekson, “Measurement of polarization mode dispersion accumulation along installed optical fibers”, IEEE Photonics Technology Letters, 11(7) pp. 860-862 (1999). but this approach may be prohibitively expensive.
The optical emission from most OTDRs that are commercially-available today is centered about one discrete wavelength, although in recent years a number of OTDRs have become available comprising several DFB lasers which can be selected individually to provide pulses at three or four different discrete wavelengths, for example 1310 nm, 1490 nm, 1550 nm and 1625 nm. However, many applications require a tunable OTDR, i.e., an OTDR whose wavelength can be set to any wavelength within a relatively broad range. See, for example, the paper by Sunnerud et al. (supra).
For example, dense wavelength-division multiplexing (DWDM) systems and coarse wavelength-division multiplexing (CWDM) systems may require an OTDR that can measure the distributed loss of a fiber at each of a plurality of ITU-grid optical frequency channels; polarization mode dispersion (PMD) measurements (e.g., polarization-sensitive OTDR, or POTDR, for distributed measurements) may require a pulsed-output laser having a wide wavelength tuning range and relatively narrow linewidth, for example, as described in recently published international patent application No. WO/2007/036051. Also, accurate single-ended chromatic dispersion (CD) measurements (CD-OTDR) using a Fresnel reflection at the end of an optical fiber link may entail measurements at wavelengths differing by as much as 100 nm.
It is possible to obtain a pulsed output of varying duration from a continuously-tunable OTDR, for example an external cavity laser (ECL), by directly modulating the gain medium. However, such lasers often exhibit an unacceptably wide laser linewidth upon direct modulation, as a result of the relatively wide spacing between adjacent longitudinal cavity modes (typically 10 GHz for a 15-mm cavity length). Also, depending upon the cavity length and the relaxation oscillation dynamics of such lasers, it may be impossible to produce optical pulses having a rise time less than 1 ns. Consequently, one frequently is obliged to use an external optical modulator or switch, for example an acousto-optical modulator (AOM), to shape the output pulse.
Optionally, an optical amplifier, for example an erbium-doped fiber amplifier (EDFA), may be deployed after the tunable laser to compensate for loss from the optical modulator (switch) in order to achieve a high optical peak power with short pulses and fast risetimes (see, for example, the paper by Sunnerud et al. (supra)). Alternatively, the EDFA/AOM combination may be replaced by a semiconductor optical amplifier (SOA), which can be modulated to produce amplified pulses and, when no current is applied, will very strongly attenuate light impinging upon it (see, for example, WO/2007/036051). A disadvantage of these “extra-cavity” optical components (EDFA, AOM and SOA) is that they are very expensive, so such a pulsed laser may not be viable for many commercial applications, for example tunable OTDRs having a widely-variable pulse duration and short risetime.